base film for membrane switch and membrane switch

ABSTRACT

There is provided a membrane switch film comprising polyethylene-2,6-naphthalenedicarboxylate as the major component and having excellent handling properties and durability, and particularly a film which is suitable for a membrane switch to be used in automobile interior devices which are potentially exposed to high temperature. The film is a membrane switch film having a refractive index in the range of 1.770-1.790 on both surfaces in either or both the film processing direction and the widthwise direction and an absolute value of no greater than 0.015 for the difference between the refractive indexes of both surfaces, or having a melting sub-peak temperature of between 220° C. and 250° C. as measured with a differential scanning calorimeter (DSC) and an absolute value of no greater than 6° C. for the difference between the melting sub-peak temperature on one surface and the melting sub-peak temperature on the other surface.

FIELD OF THE INVENTION

The present invention relates to a membrane switch base film composed ofa biaxial oriented polyester film comprisingpolyethylene-2,6-naphthalenedicarboxylate as the major component. Morespecifically, it relates to a membrane switch base film with excellenthandling properties and durability, and especially to a membrane switchbase film which is suitable for use in automobile interior devices whichare potentially exposed to high temperature.

BACKGROUND ART

A membrane switch comprises two base films lying on either side of aspacer, with contact points (electrodes) corresponding to each of theopposing surfaces. Switching action between conduction and insulationcan be easily accomplished by pressing the base films, i.e. varying thespacing between the base films. Membrane switches have recently comeinto wide use for the keypads of cellular phones or portable personalcomputers or the various control panel switches of household electricalappliances such as VTRs and microwave ovens.

Because the switching action of a membrane switch involves repeatedpressing, the base films used therefor must be flexible and resistant todeformation. Base films used for conventional membrane switches havegenerally been polyethylene terephthalate (hereinafter also abbreviatedas “PET”) films because of their deformation resistance, adhesion withelectrodes, bonding with printing pastes, and other properties.

Recently, however, membrane switches and remote control switches arebecoming more common in the operating panels of automobile-related parts(car audio systems, car air conditioners, car navigation systems and thelike). Base films of membrane switches have therefore been required tohave deformation resistance at high temperatures that cannot bewithstood by PET films. For example, vehicle interior temperatures areknown to reach approximately 80° C. during the daytime in summer, andsuch interior temperatures often exceed the glass transition point (Tg)of PET. When a PET film is used as a membrane switch base film in suchan environment, the high temperature burden results in deformation ofthe PET film which remains even after the burden no longer exists, andsuch warping of the PET film is a cause of switch malfunction.

As a means of improving this situation, Japanese Examined PatentPublication HEI No. 4-75610 has proposed using as the base filmspolyethylene naphthalenedicarboxylate (hereinafter also abbreviated as“PEN”) films which have a higher glass transition point than PET films.Specifically, Japanese Examined Patent Publication HEI No. 4-75610discloses a membrane switch having contact points corresponding to eachof the opposing surfaces of two base films, which employs as at leastone of the base films a biaxial oriented polyethylenenaphthalenedicarboxylate film having an F-5 value (5% elongation stress)of ≧11 kg/mm², a density of ≦1.375 g/cm³ and a thermal shrinkage factorof ≦1.0% when heated at 120° C. for 30 minutes.

Also, Japanese Examined Patent Publication HEI No. 6-4276 disclosespolyester film for a membrane switch composed of polyethylenenaphthalate having a haze increase rate of no greater than 20% asdefined by the following formula upon heat treatment at 150° C. for 2hours:Haze increase rate={(H2−H1)/H1}×100 (%)(where H1 is the haze value before heat treatment and H2 is the hazevalue after heat treatment),and a thermal shrinkage factor of no greater than 0.5% in both the filmprocessing direction (the direction of propagation of the film duringcontinuous film processing, also referred to as the film longitudinaldirection, machine direction, continuous film formation direction or MDdirection) and the widthwise direction (the direction orthogonal to thefilm forming direction within the plane of the film, also referred to asthe transverse direction or TD).

DISCLOSURE OF THE INVENTION

Recently, however, more stringent quality has been required for membraneswitch base films, and even some PEN films have been inadequate in termsof deformation resistance or durability.

In response to the changing requirements for quality, it has beendisclosed in Domestic Republished Application 99/37466 that deformationresistance can be improved by specific heat treatment of PEN films.However, for handling in the processing of the membrane switch by themethod described in this publication, there are restrictions on the heattreatment temperature before and after circuit printing. It hastherefore been desired to obtain a membrane switch base film whichexhibits excellent durability while also having superior handleabilityin the process.

It is a first object of the present invention to solve theaforementioned problems of the prior art by providing a base film havingexcellent film handleability and durability, which is particularlysuitable for a membrane switch to be used for automobile interiordevices which are potentially exposed to high temperatures. It isanother object of the invention to provide a membrane switch base filmwith excellent workability for punching out membrane switch shapes fromthe film.

Other objects and advantages of the invention will become readilyapparent by the following description.

According to the invention, the aforementioned objects and advantagesare achieved, firstly, by:

-   -   a membrane switch base film composed of a biaxially oriented        polyester film comprising        polyethylene-2,6-naphthalenedicarboxylate as the major        component, the film having a refractive index in the range of        1.770-1.790 on both surfaces in either or both the film        processing direction and the widthwise direction and an absolute        value of no greater than 0.015 for the difference between the        refractive indexes of both surfaces.

According to the invention, the aforementioned objects and advantagesare achieved, secondly, by:

-   -   a membrane switch base film composed of a biaxially oriented        polyester film comprising        polyethylene-2,6-naphthalenedicarboxylate as the major        component, the film having a melting sub-peak temperature of        between 220° C. and 250° C. as measured with a differential        scanning calorimeter (DSC) and an absolute value of no greater        than 6° C. for the difference between the melting sub-peak        temperature on one surface and the melting sub-peak temperature        on the other surface.

According to the invention, the aforementioned objects and advantagesare achieved, thirdly, by:

-   -   a membrane switch base film composed of a biaxially oriented        polyester film comprising        polyethylene-2,6-naphthalenedicarboxylate as the major        component, the film having (1) a refractive index in the range        of 1.770-1.790 on both surfaces in either or both the film        processing direction and the widthwise direction and an absolute        value of no greater than 0.015 for the difference between the        refractive indexes of both surfaces, and (2) a melting sub-peak        temperature of between 220° C. and 250° C. as measured with a        differential scanning calorimeter (DSC) and an absolute value of        no greater than 6° C. for the difference between the melting        sub-peak temperature on one surface and the melting sub-peak        temperature on the other surface.

According to the invention, the aforementioned objects and advantagesare achieved, fourthly, by:

-   -   a membrane switch comprising a membrane switch base film        according to the first, second or third aspect of the invention,        a spacer and electrodes.

DETAILED DESCRIPTION OF THE INVENTION

<Polyethylene-2,6-naphthalenedicarboxylate>

The polymer composing the biaxially oriented polyester film of theinvention comprises polyethylene-2,6-naphthalenedicarboxylate(hereinafter also abbreviated as “PEN”) as the major component, and itmay be also be a copolymer or blend. The term “major” as used in thepresent invention means that ethylene-2,6-naphthalenedicarboxylateconstitutes at least 80 mole percent, preferably at least 90 molepercent and more preferably at least 95 mole percent of the totalrepeating units of the polymer. It is only necessary to guaranteepermanent deformation resistance with use under high temperature,without extreme loss of the original properties of the biaxiallyoriented polyester film of the invention.

In the case of a copolymer, compounds having two ester-formingfunctional groups in the molecule may be used as the copolymerizingcomponents of the copolymer other thanpolyethylene-2,6-naphthalenedicarboxylate as the major component. Assuch compounds there may be used, preferably, dicarboxylic acids such asoxalic acid, adipic acid, phthalic acid, sebacic acid,dodecanedicarboxylic acid, isophthalic acid, terephthalic acid,1,4-cyclohexanedicarboxylic acid, 4,4′-diphenyldicarboxylic acid,phenylindanedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,tetralindicarboxylic acid, decalindicarboxylic acid anddiphenyletherdicarboxylic acid. There may also be suitably usedoxycarboxylic acids such as p-oxybenzoic acid and p-oxyethoxybenzoicacid. In addition, there may be suitably used dihydric alcohols such aspropylene glycol, trimethylene glycol, tetramethylene glycol,hexamethylene glycol, cyclohexanemethylene glycol, neopentyl glycol,bisphenolsulfone ethylene oxide addition product, bisphenol A ethyleneoxide addition product, diethylene glycol and polyethylene oxide glycol.

These compounds may be used alone or in combinations of two or more.Among these compounds are preferred those with isophthalic acid,terephthalic acid, 4,4′-diphenyldicarboxylic acid,2,7-naphthalenedicarboxylic acid and p-oxybenzoic acid as acidcomponents and trimethylene glycol, hexamethylene glycol, neopentylglycol and bisphenolsulfone ethylene oxide addition product as glycolcomponents.

The PEN used for the invention may also have terminal hydroxyl and/orcarboxyl groups, all or a portion of which are blocked with amonofunctional compound such as benzoic acid or methoxypolyalkyleneglycol. Alternatively, the PEN used for the invention may becopolymerized with, for example, a trace amount of a tri- or higherfunctional ester-forming compound such as glycerin, pentaerythritol orthe like, in a range which gives a substantially linear polymer.

The polymer composing the membrane switch base film of the invention mayalso be, instead of PEN, a blend with other organic polymer. As organicpolymers blended with PEN there may be mentioned polyethyleneterephthalate, polyethylene isophthalate, polytrimethyleneterephthalate, polyethylene-4,4′-tetramethylenediphenyldicarboxylate,polyethylene-2,7-naphthalenedicarboxylate,polytrimethylene-2,6-naphthalenedicarboxylate,polyneopentylene-2,6-naphthalenedicarboxylate,poly(bis(4-ethyleneoxyphenyl)sulfone)-2,6-naphthalenedicarboxylate andthe like. Among these, polyethylene isophthalate, polytrimethyleneterephthalate, polytrimethylene-2,6-naphthalenedicarboxylate andpoly(bis(4-ethyleneoxyphenyl)sulfone)-2,6-naphthalenedicarboxylate areparticularly preferred.

These organic polymers to be blended with PEN may be used alone or incombinations of two or more. The proportion of an organic polymer to beblended with PEN may be in the range of no more than 20 mole percent,preferably no more 10 mole percent and more preferably no more than 5mole percent of the repeating units of the polymer. Production of such ablend can be accomplished by a commonly known process for production ofpolyester composition.

The polyester which is comprised by the base film according to theinvention may be obtained by a publicly known process of the prior art.For example, a low polymerization degree polyester may be obtaineddirectly by reaction between a dicarboxylic acid and glycol, ortransesterification of a dicarboxylic acid lower alkyl ester and glycolmay be carried out using a publicly known transesterification catalystof the prior art, followed by polymerization reaction in the presence ofa polymerization catalyst.

As examples of transesterification catalysts there may be mentionedcompounds containing sodium, potassium, magnesium, calcium, zinc,strontium, titanium, zirconium, manganese and cobalt, of which any oneor combination of two or more may be used. As polymerization catalyststhere may be mentioned antimony compounds such as antimony trioxide andantimony pentaoxide, germanium compounds such as germanium dioxide, andtitanium compounds such as tetraethyl titanate, tetrapropyl titanate,tetraphenyl titanate or their partial hydrolysates, ammonium titanyloxalate, potassium titanyl oxalate and titanium trisacetylacetonate.

For polymerization via transesterification, a phosphorus compound suchas trimethyl phosphate, triethyl phosphate, tri-n-butyl phosphate ororthophosphoric acid may be added in order to inactivate thetransesterification catalyst prior to polymerization reaction. Thecontent of such a phosphorus compound is preferably 20-100 ppm by weightin terms of phosphorus element in the PEN, from the standpoint ofthermal stability of the polyester. The polyester may be into chipsafter melt polymerization, then the chips may be polymerized in solidphase with heating under reduced pressure or under an inert gas streamof nitrogen or the like.

The intrinsic viscosity of the polyester comprising PEN as the majorcomponent is preferably from 0.40 dl/g to 0.90 dl/g. It is morepreferably 0.43-0.85 dl/g and most preferably 0.45-0.80 dl/g. If theintrinsic viscosity is below this limit the film may be brittle, tendingto result in tearing of the film during its processing. If the intrinsicviscosity of the film is above this limit, the intrinsic viscosity ofthe polymer will need to be rather high, and this may require a longertime for polymerization by ordinary synthesis methods and thus impairproductivity. The intrinsic viscosity is the value (units: dl/g)measured at 35° C. using o-chlorophenol as the solvent.

<Additives>

The membrane switch film of the invention preferably contains a smallproportion of inert particles to provide slipperiness to the film. Asexamples of such inert particles there may be mentioned inorganicparticles such as spherical silica, porous silica, calcium carbonate,alumina, titanium dioxide, kaolin clay, barium sulfate and zeolite, ororganic particles such as silicone resin particles and crosslinkedpolystyrene particles. Inorganic particles are preferably syntheticparticles rather than natural particles, because the particle sizes ofthe former are more uniform. The crystal form, hardness, specificgravity and color of the inorganic particles is not particularlyrestricted, and may be selected depending on the purpose of use.

As specific inorganic particles there may be mentioned calciumcarbonate, porous silica, spherical silica, kaolin, talc, magnesiumcarbonate, barium carbonate, calcium sulfate, barium sulfate, lithiumphosphate, calcium phosphate, magnesium phosphate, aluminum oxide,silicon oxide, titanium oxide, zirconium oxide, lithium fluoride and thelike. Particularly preferred among these are calcium carbonateparticles, spherical silica particles, porous silica particles andlaminar aluminum silicate.

As organic particles there may be mentioned organic salt particles,crosslinked polymer particles and the like. As examples of organic saltparticles there may be mentioned calcium oxalate or terephthalic acidsalts of calcium, barium, zinc, manganese, magnesium and the like. Asexamples of crosslinked polymer particles there may be mentionedhomopolymers or copolymers of vinyl-based monomers such asdivinylbenzene, styrene, acrylic acid and methacrylic acid. There mayalso be mentioned preferred organic particles such aspolytetrafluoroethylene, silicone resins, benzoguanamine resins,thermosetting epoxy resins, unsaturated polyester resins, thermosettingurea resins and thermosetting phenol resins. Particularly preferredamong these crosslinked polymer particles are silicone resin particlesand crosslinked polystyrene particles.

The particle size of inert particles added to the film is preferably amean particle size of 0.05-5 μm, more preferably 0.08-3.5 μm and evenmore preferably 0.10-3 μm for each type of particle. The total amount ofinert particles added to the film is preferably 0.05-3 wt %, morepreferably 0.08-2.5 wt % and even more preferably 0.1-2.0 wt %.

The inert particles added to the film may consist of one componentselected from among those mentioned above, or a plural components, suchas two or three components. In the case of a single component, two ormore types of particles with different mean particle sizes may be used.

The mean particle size of the inert particles is the value of theparticle size corresponding to 50 wt % on a integrating curve forparticles having different particle sizes measured using a CP-50Centrifugal Particle Size Analyzer by Shimazu Corp. and calculated basedon a centrifugal sedimentation curve, plotted against their contents(see “Ryudo Sokutei Gijutsu” [Particle Size Measuring Techniques],Nikkan Kogyo Shinbun Co., Ltd., 1975, pp. 242-247).

Most preferably, the base film of the invention comprises from 0.1 wt %to 0.4 wt % of calcium carbonate particles with a mean particle size of0.3-0.8 μm and/or from 0.03 wt % to 0.5 wt % of spherical silicaparticles with a mean particle size of 0.1-0.6 μm and/or from 0.03 wt %to 0.4 wt % of silicone particles with a mean particle size of 0.1-0.6μm. The same type of inert fine particles with different particle sizesmay also be used simultaneously, in which case the total content ofinert particles of the same type may be within the ranges specifiedabove.

The base film of the invention may also contain a crystal nucleatingagent, antioxidant, heat stabilizer, lubricant, flame retardant,antistatic agent, polysiloxane or the like, depending on the purpose ofuse.

The timing for addition of the inert particles and other additives isnot particularly restricted so long as it is at a stage up to processingof the film of the polyester with PEN as the major component, and forexample, they may be added at the polymerization stage or duringprocessing of the film. From the viewpoint of achieving uniformdispersion, the inert particles and other additives are preferably addedin ethylene glycol and added to a high concentration duringpolymerization to prepare master chips, and the master chipssubsequently diluted with unadded chips.

<Difference Between Melting Sub-Peak Temperatures (Tsm) of Front andBack Sides of Base Film>

The melting sub-peak temperature (Tsm) of the base film of the inventionas measured with a differential scanning calorimeter (DSC) must be from220-250° C. Also, an additional condition is that the absolute value ofthe difference between the melting sub-peak temperatures (Tsm) of oneside (designated as the “front”) and the other side (designated as the“back”) of the film (|Tsm (front)−Tsm (back)|) must be no greater than6° C.

If the melting sub-peak temperature is below the lower limit, finecracks or burrs will tend to be formed at the edges of the film when itis cut into a sheet. On the other hand, if the melting sub-peaktemperature is above the upper limit, the film will lose toughness andthe durability of the switch will thus be impaired. Also, if theabsolute value of the difference between the melting sub-peaktemperatures (Tsm) of the front and back sides of the film (|Tsm(front)−Tsm (back)|) is greater than 6° C., fine cracks or burrs willtend to be formed at the edges of the film when it is cut into a sheet,or the film will tend to become curled after being rolled and: storedfor a period as a roll.

The melting sub-peak temperature (Tsm) is more preferably from 225° C.to 245° C. and even more preferably from 230° C. to 245° C. The absolutevalue of the difference between the melting sub-peak temperatures (Tsm)of the front and back sides of the film (|Tsm (front)−Tsm(back)|) ismore preferably no greater than 5° C. and even more preferably nogreater than 4° C.

<Refractive Index of Base Film>

The refractive index in either or both the film processing direction andthe widthwise direction in each plane of the film of the invention mustbe from 1.770 to 1.790, and more preferably from 1.772 to 1.788.According to the invention, unless otherwise specified, the filmprocessing direction is the direction of propagation of the film duringcontinuous film processing, and it is also referred to as the filmlongitudinal direction, machine direction, continuous film processingdirection or MD direction. Also according to the invention, thewidthwise direction is the direction orthogonal to the film processingdirection within the plane of the film, and it is also referred to asthe transverse direction or TD. If the refractive indexes of the filmprocessing direction and widthwise direction of the film are both belowthe aforementioned lower limits, the durability of the film will beimpaired. On the other hand, if the refractive indexes of the filmprocessing direction and widthwise direction of the film are both abovethe aforementioned upper limits, the film will tend to tear frequentlyduring its production. The refractive index of the film was measured onboth sides using a laser refractometer (measuring wavelength: 633 nm)based on the Abbe refractometer principle.

<Difference Between Refractive Indexes of Front and Back Sides of BaseFilm>

The absolute value of the difference between the refractive index of oneside (designated as the “front”) and the refractive index of the otherside (designated as the “back”) of the film (|refractive index(front)−refractive index (back)|) must be no greater than 0.015 in atleast one direction of the base film of the invention in which therefractive index is from 1.770 to 1.790. The absolute value is morepreferably no greater than 0.013 and even more preferably no greaterthan 0.011. If the absolute value of the difference between therefractive indexes of the front and back sides of the film exceeds 0.015in at least one direction in which the refractive index is from 1.770 to1.790, fine cracks or burrs will tend to be formed at the edges of thefilm when it is cut into a sheet, or the film will tend to become curledafter being rolled and stored for a period as a roll.

<Refractive Index in Widthwise Direction of Base Film>

The refractive index in the widthwise direction on each side of the basefilm of the invention is preferably from 1.770 to 1.790, and morepreferably from 1.772 to 1.788. The refractive index was measured oneach side using a laser refractometer (measuring wavelength: 633 nm)based on the Abbe refractometer principle. If the refractive index inthe widthwise direction of the film is below the aforementioned lowerlimit, the durability of the film may be impaired. On the other hand, ifthe refractive index of the film is above the upper limit, the film willtend to tear frequently during its production.

<Phosphorus Compound and Titanium Compound in Base Film>

As mentioned above, the base film of the invention preferably contains aphosphorus compound. As suitable phosphorus compounds there may bementioned phosphoric acid, phosphorus acid, phosphonic acid, phosphonatecompounds and their derivatives, of which any one alone or combinationof two or more may be used. Preferred among phosphorus compounds arephosphonate compounds represented by the following formula (I).R¹O—C(O)—X—P(O)—(OR²)₂  (I)where R¹ and R² represent C₁₋₄ alkyl groups and X represents —CH₂— or—CH(Y)— (where Y represents phenyl), and R¹ and R² may be the same ordifferent.

Particularly preferred phosphorus compounds include dimethyl esters,diethyl esters, dipropyl esters and dibutyl esters ofcarbomethoxymethanephosphonic acid, carboethoxymethanephosphonic acid,carbopropoxymethanephosphonic acid, carbobutoxymethanephosphonic acid,carbomethoxy-phosphono-phenylacetic acid,carboethoxy-phosphono-phenylacetic acid,carbopropoxy-phosphono-phenylacetic acid andcarbobutoxy-phosphono-phenylacetic acid.

These phosphonate compounds are preferred according to the inventionbecause they promote reaction with titanium compounds more smoothly thanphosphorus compounds which are commonly used as stabilizers, and becausethey lengthen the duration of the catalytic activity of titaniumcompounds so that a smaller amount thereof may be added to catalyze thepolyester, while the thermal stability of the polyester is not impairedas easily even with a large amount of stabilizer added to the catalyst.

The timing for addition of the phosphorus compound may be as desired solong as it is after the transesterification reaction has substantiallybeen completed, and for example, the phosphorus compound may be added atatmospheric pressure before start of the polycondensation reaction,under reduced pressure after start of the polycondensation reaction,during the final stage of the polycondensation reaction or aftercompletion of the polycondensation reaction, i.e. after obtaining thepolymer.

According to the invention, the catalyst used for production of the PENis preferably a titanium compound which is substantially soluble in PEN,from the standpoint of minimizing contaminants from the catalyst.Specifically, the contents of antimony element or germanium elementoriginating in an antimony compound or germanium compound commonly usedas a catalyst in the prior art is preferably a maximum of 5 mmol % basedon the number of moles of the ethylene-2,6-naphthalenedicarboxylatecomponent. An antimony element or germanium element content exceeding 5mmol % result in problems such as precipitation of contaminants from thecatalyst.

The titanium compound is not particularly restricted so long as it issoluble in the polymer, and as examples of common titanium compounds aspolyester polycondensation catalysts there may be mentioned titaniumacetate, tetra-n-butoxytitanium, and the like. Among these, compoundsrepresented by the following formula (II) or reaction products ofcompounds represented by formula (II) and aromatic polyvalent carboxylicacids represented by the following formula (III) or their anhydrides,are preferred.Ti(OR³)(OR⁴)(OR⁵)(OR⁶)  (II)where R³, R R⁵ and R⁶ each represent C₂₋₁₀ alkyl and/or phenyl.

where n represents an integer of 2 to 4.

The titanium tetraalkoxides represented by formula (II) above are notparticularly restricted so long as R³, R⁴, R⁵ and R⁶ each representC₂₋₁₀ alkyl and/or phenyl. Particularly preferred titaniumtetraalkoxides represented by formula (II) includetetraisopropoxytitanium, tetrapropoxytitanium, tetra-n-butoxytitanium,tetraethoxytitanium, tetraphenoxytitanium, and the like. As aromaticpolyvalent carboxylic acids represented by formula (III) there arepreferred phthalic acid, trimellitic acid, hemimellitic acid andpyromellitic acid. The aromatic polyvalent carboxylic acids representedby general formula (III) may also be their corresponding anhydrides. Forreaction between the titanium-compound and the aromatic polyvalentcarboxylic acid, a portion of the aromatic polyvalent carboxylic acid orits anhydride is dissolved in a solvent and the titanium compound isadded dropwise thereto for reaction at a temperature of 0-200° C. for 30minutes or longer.

The content of the titanium compound used as the catalyst is preferablyfrom 4 mmol % to 15 mmol % in terms of titanium element based on thenumber of moles of the ethylene-2,6-naphthalenedicarboxylic acid. It ismore preferably from 6 mmol % to 12 mmol %, and even more preferablyfrom 6 mmol % to 10 mmol %. If the content of the titanium compound isbelow this lower limit, the PEN productivity will be lower, making itdifficult to obtain PEN with the desired molecular weight. If thecontent of the titanium compound is above the upper limit, the resultingPEN will tend to have lower thermal stability and the molecular weightmay be significantly reduced upon melt extrusion during production ofthe film. The PEN-soluble titanium compound content referred to here isthe total of the titanium compound used as the transesterificationcatalyst and the titanium compound used as the polycondensationcatalyst, when the polymerization is conducted via transesterification.

The base film of the invention preferably employs the aforementionedtitanium compound as a catalyst in the production stage of the resincomposition, and comprises a phosphorus compound as a stabilizer. Thecontents of the titanium compound and phosphorus compound preferablysatisfy the following inequalities (1) to (3) in addition to theconditions stated above.4≦Ti≦15  (1)0.5≦P/Ti≦15  (2)15≦Ti+P≦150  (3)

In inequalities (1) to (3), Ti is the value of the number of moles oftitanium element in the titanium compound divided by the number of molesof the ethylene-2,6-naphthalenedicarboxylate component in thecomposition (mmol %), and P is the value of the number of moles ofphosphorus element in the phosphorus compound divided by the number ofmoles of the ethylene-2,6-naphthalenedicarboxylate component in thecomposition (mmol %).

If (P/Ti) is below the lower limit, the resulting PEN will tend to havelower thermal stability and heat degradation products will oftenprecipitate around the die slit, resulting in contamination of the slitopening or the peripheral sections of the die. On the other hand, if(P/Ti) is above the upper limit, the reactivity during polymerization ofthe PEN will be vastly reduced, often making it difficult to obtain PENwith the desired molecular weight. A more preferred range for (P/Ti) isfrom 2 to 10.

If (Ti+P) is below the lower limit, the productivity may be reduced forfilm processing based on electrostatic printing, and the uniformity ofthe film thickness may be impaired. On the other hand, if (Ti+P) isabove the upper limit, contaminants from the catalyst may tend to begenerated, although in small amounts, and the catalyst-derivedcontaminants will tend to precipitate around the die slit of the meltextruder during production of the film, causing striped surface defectsalong the film processing direction. A more preferred range for (Ti+P)is from 25 to 100.

The PEN of the invention may employ 2,6-naphthalenedicarboxylic acid andethylene glycol as the starting materials, or it may employ a2,6-naphthalenedicarboxylic acid ester-forming derivative such as2,6-dimethyl naphthalate and ethylene glycol as the starting materials.Preferred is a production process via transesterification wherein atleast 80 mole percent of the total dicarboxylic acid component used asthe starting material is 2,6-dimethyl naphthalate. More preferred amongproduction processes using 2,6-dimethyl naphthalate as the startingmaterial is a process wherein at least a portion of the titaniumcompound is added prior to the start of transesterification, for use asboth the transesterification catalyst and the polycondensation catalyst,because this will reduce the amount of titanium compound to be added.The transesterification reaction is preferably carried out underpressurization of from 0.05 MPa to 0.20 MPa in order to further reducethe amount of titanium compound added.

<Thermal Shrinkage>

The thermal shrinkage factor of the base film of the invention in thefilm processing direction and widthwise direction when it is subjectedto heat treatment at 200° C. for 10 minutes is preferably-between 0.2%and 1.4%, and more preferably between 0.3% and 1.3%. If the thermalshrinkage factor upon heat treatment at a temperature of 200° C. for 10minutes is above the upper limit, dimensional variation may increase andthe flatness of the film may be impaired by the preheating treatmentbefore working into a membrane switch. On the other hand, if the thermalshrinkage factor is below the lower limit, the durability of theobtained membrane switch may be impaired.

There are no particular restrictions on the difference in thermalshrinkage factor in the film processing direction and widthwisedirection (thermal shrinkage factor (MD)−thermal shrinkage factor (TD))after heat treatment at 200° C. for 10 minutes, but in order to preventflatness impairment, the difference in thermal shrinkage factor in thefilm processing direction and the widthwise direction is preferablybetween −1.0% and 0.5%.

<Base Film Thickness>

The thickness of the membrane switch base film of the invention ispreferably from 40 μm to 190 μm, more preferably from 45 μm to 175 μmand even more preferably from 50 μm to 160 μm. If the film thickness isbelow the lower limit, the durability for repeated pressing may beinadequate. On the other hand, if the film thickness is above the upperlimit, the film will be too resistant to flexing for use as a membraneswitch.

Variation in the thickness of the film of the invention at any givenlocation is preferably no greater than 10% and more preferably nogreater than 8% as compared with the center thickness. A smallervariation in film thickness is more preferable for stable operation of amembrane switch.

<Base Film Roughness (SRa)>

The roughness, and specifically the three-dimensional center planeaverage roughness (SRa), of the membrane switch base film of theinvention on at least one side is preferably from 10 nm to 45 nm. It ismore preferably from 10 nm to 40 nm and even more preferably from 12 nmto 35 nm. If the SRa is below the lower limit, films stacked together inlarge amounts will have poor slipperiness with each other when each filmis fed out in order during the switch fabrication process, thusresulting in feed failures. On the other hand, if the SRa is above theupper limit, films stacked together in large amounts will have excessiveslipperiness with each other, often resulting in frequent slippageduring film stacking.

<Base Film Density>

The density of the base film of the invention is preferably from 1.350g/cm³ to 1.367 g/cm³. It is more preferably from 1.352 g/cm³ to 1.365g/cm³ and even more preferably from 1.354 g/cm³ to 1.363 g/cm³. If thedensity is below the lower limit, the durability for repeated pressingmay be impaired. If it is above the upper limit, the crystallinity maybe too high, potentially resulting in loss of film toughness and thuspoor workability of the membrane switch. The film density is the valueas measured by the suspension method at 25° C. in a density gradienttube using aqueous calcium nitrate as the solvent.

<Coating Layer>

The base film of the invention may also be provided with a coating layeron at least one side for the purpose of enhancing adhesion with printingpastes.

The coating layer preferably comprises at least one type ofwater-soluble or water-dispersible polymer resin selected from amongpolyester resins, urethane resins, acrylic resins and vinyl-basedresins, with combinations of polyester resins and acrylic resins beingparticularly preferred. A polyester resin in a coating layer used forthe invention has a glass transition point (Tg) of 0-100° C. and morepreferably 10-90° C. Such a polyester resin is preferably a polyesterwhich is soluble or dispersible in water, but it may also contain someamount of an organic solvent.

Such polyester resins may comprise the following polybasic acids ortheir ester-forming derivatives and polyols or their ester-formingderivatives. As polybasic acid components there may be mentionedterephthalic acid, isophthalic acid, phthalic acid, phthalic anhydride,2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid,adipic acid, sebacic acid, trimellitic acid, pyromellitic acid, dimeracid, 5-sodiumsulfoisophthalic acid, and the like. Two or more of theseacid components may also be used for synthesis of copolymer polyesterresins. There may also be used, in slight amounts, unsaturated polybasicacid components such as maleic acid and itaconic acid, orhydroxycarboxylic acids such as p-hydroxybenzoic acid. As polyolcomponents there may be mentioned ethylene glycol, 1,4-butanediol,diethylene glycol, dipropylene glycol, 1,6-hexanediol,1,4-cyclohexanedimethanol, xylene glycol, dimethylolpropane,poly(ethyleneoxide) glycol, poly(tetramethyleneoxide) glycol, bisphenolA, bisphenol A ethylene oxide or propylene oxide addition products, andthe like. The polyester resin used as a coating layer may be formed fromsuch representative monomers, but is not limited thereto.

An acrylic resin in a coating layer used for the invention has a glasstransition point (Tg) of −50 to 50° C. and more preferably −50 to 25° C.Such an acrylic resin is preferably an acrylic resin which is soluble ordispersible in water, but it may also contain some amount of an organicsolvent. Such acrylic resins may be copolymerized from the followingacrylic monomers. As acrylic monomers there may be mentioned alkylacrylates and alkyl methacrylates (with methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, t-butyl, 2-ethylhexyl, cyclohexyl or thelike as alkyl groups); hydroxy group-containing monomers such as2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropylacrylate and 2-hydroxypropyl methacrylate; epoxy group-containingmonomers such as glycidyl acrylate, glycidyl methacrylate andallylglycidyl ether; monomers containing carboxyl groups or their salts,such as acrylic acid, methacrylic acid, itaconic acid, maleic acid,fumaric acid, crotonic acid, styrenesulfonic acid and their salts(sodium salts, potassium salts, ammonium salts, tertiary amine salts andthe like); amide-containing monomers such as acrylamide, methacrylamide,N-alkylacrylamide, N-alkylmethacrylamide, N,N-dialkylacrylamide,N,N-dialkyl methacrylate (with methyl, ethyl, n-propyl, isopropyl,n-butyl, isobutyl, t-butyl, 2-ethylhexyl, cyclohexyl or the like asalkyl groups), N-alkoxyacrylamide, N-alkoxymethacrylamide,N,N-dialkoxyacrylamide, N,N-dialkoxymethacrylamide (with methoxy,ethoxy, butoxy, isobutoxy or the like as alkoxy groups),acryloylmorpholine, N-methylolacrylamide, N-methylolmethacrylamide,N-phenylacrylamide and N-phenylmethacrylamide; acid anhydride monomerssuch as maleic anhydride and itaconic anhydride; and other monomers suchas vinyl isocyanate, allyl isocyanate, styrene, α-methylstyrene,vinylmethyl ether, vinylethyl ether, vinyltrialkoxysilane, alkylmaleicacid monoesters, alkylfumaric acid monoesters, alkylitaconic acidmonoesters, acrylonitrile, methacrylonitrile, vinylidene chloride,ethylene, propylene, vinyl chloride, vinyl acetate and butadiene. Theacrylic resin used as a coating layer is not restricted to thosementioned above.

The above mentioned composition used for the invention is preferably inthe form of an aqueous coating liquid such as an aqueous solution,aqueous dispersion or emulsion for formation of the coating. Ifnecessary for formation of the coating, there may also be added otherresins in addition to the composition described above, such as oxazolinegroup-containing polymers, crosslinking agents such as melamine, epoxyor aziridine, antistatic agents, coloring agents, surfactants,ultraviolet absorbers, lubricants (filler, waxes) and the like. Alubricant may be added as necessary for the purpose of enhancing thesliding property or enhancing the blocking resistance of the film.

The solid concentration of the aqueous coating liquid will normally beno greater than 20 wt % and preferably from 1-10 wt %. If the proportionis less than 1 wt %, the polyester film coatability may be inadequate,while if it is above the upper limit, the stability of the coating agentand the coated appearance may be impaired.

The coating layer may be anchored on the film by first applying theaqueous coating liquid onto the unstretched or uniaxially stretchedfilm, and then stretching the film in one or two directions and heatsetting it. The application method may be roll coating, gravure coating,roll-brush coating, spraying, air knife coating, impregnation, curtaincoating or the like, either alone or in various combinations.

<Production Conditions>

The membrane switch base film of the invention is a biaxially orientedfilm with PEN as the major component. To obtain a biaxially orientedfilm, an ordinary method may be employed, for example, for melting atabove the melting point of the polymer and extrusion through a die slitonto a casting drum adjusted to near 60° C. to accomplish adhesion,cooling and solidification, and thereby obtain an unstretched film. Theunstretched film may then be biaxially stretched in the machine andtransverse directions and then heat set and, if necessary, subjected torelaxation treatment in the machine and/or transverse directions. Thestretching of the film may be carried out using a publicly knownroll-type longitudinal stretching machine, infrared heating longitudinalstretching machine or tenter clip-type transverse stretching machine, ora multistage stretching machine which carries out such stretching inseparate multiple stages, a tubular stretching machine, oven-typelongitudinal stretching machine or simultaneous biaxial stretchingmachine, with no particular restriction to these. According to theinvention, either sequential biaxial stretching or simultaneous biaxialstretching may be employed, so long as the refractive index can becontrolled in the range of 1.770-1.790 in either or both the filmprocessing direction and the widthwise direction.

A process for production of a polyester film of the invention will nowbe explained in detail, but without placing any restrictions on theinvention.

Simultaneous biaxial stretching will be explained first. Simultaneousbiaxial stretching machines with a longitudinal stretching mechanisminclude conventional screw-type machines wherein a clip is mounted inthe screw groove and the clip spacing is widened, pantograph-typemachines wherein a pantograph is used to widen the clip spacing, andlinear motor-type machines which utilize a linear motor. A linearmotor-type machine is preferred over a screw-type or pantograph-typemachine because the film forming speed is faster and conditions such asthe stretching factor can be varied more easily. Simultaneous biaxialstretching offers the advantage of minimal damage to the film surfacesince no longitudinal stretching rollers are used as are used forsequential biaxial stretching. Another advantage is that it is generallyeasier to control the orientation in the machine and transversedirections than with sequential biaxial stretching, because theunstretched film is simultaneously stretched in the machine andtransverse directions. These features are suited for the propertiesrequired of a membrane switch film of the invention, and thereforesimultaneous biaxial stretching may be employed for the invention.

According to the invention, simultaneous biaxial stretching refers tostretching for orientation simultaneously in the machine and transversedirections of the film, and it is an operation whereby a simultaneousbiaxial stretching machine is used to convey the film while clampingboth edges with a clip in order to accomplish stretching in the machineand transverse directions. Needless to mention, it is sufficient if partof the stretching in the machine and transverse directions is carriedout simultaneously, and therefore the scope of the invention includesmethods in which the film is first stretched in the transverse directionor machine direction and then simultaneously stretched in the machinedirection and transverse direction, as well as methods in whichsimultaneous biaxial stretching is followed by stretching in thetransverse direction or machine direction alone.

For production of a film according to the invention, inert particles areadded to the prescribed PEN and then a molten film of the PEN which hasbeen melt extruded at a temperature of, for example, 280-330° C. israpidly cooled on the surface of a rotating cooling drum set to asurface temperature of 30-70° C., to obtain an unstretched film with anintrinsic viscosity of 0.40-0.90 dl/g. The drying prior to meltextrusion is preferably conducted for 4-7 hours at 160-190° C. The ratioof the thickness at the edge and the center of the unstretched film(edge thickness/center thickness) is preferably between 1 and 10, morepreferably at least 1 and less than 5 and even more preferably at least1 and less than 3. A thickness ratio of less than 1 or greater than theupper limit is not preferred as it may increase the incidence of filmtearing or clip slipping.

The unstretched film is then guided into a simultaneous biaxialstretching machine with both edges of the film held by the clip and isheated to 80-170° C. in a preheating zone, after which it is subjectedto simultaneous biaxial stretching at 120-170° C., in one stage or intwo or more stages, to an area increase factor of 9-20 (2-4.5longitudinally). If necessary, it may be subsequently subjected tosimultaneous biaxial stretching in a temperature range of 140-245° C.,in one stage or in two or more stages, to an area increase factor of2-5. Next, the film is heat set in a temperature range of 190-250° C.and if necessary, subjected to relaxation treatment either during theheat setting or in the cooling process after heat setting. Therelaxation treatment is preferably conducted in a temperature range of140-240° C., with relaxation preferably in a range of 1-10% in both themachine and transverse directions. For the membrane switch film of theinvention, the preheating temperature is preferably about 130° C., thestretching temperature about 145° C. and the heat setting temperatureabout 2.40° C., and if necessary, relaxation treatment is carried out inthe machine and transverse directions during the heat setting, afterwhich the film is cooled to room temperature and rolled to obtain thesimultaneous biaxial stretched film. According to the invention, thepolyester film surface is preferably coated with a coating agent eitherbefore or after the simultaneous biaxial stretching in order to impartfunctions such as adhesive property, sliding property, release propertyand electrostatic property to the surface of the film.

The film of the invention may also be produced by ordinary sequentialbiaxial stretching. A PEN unstretched film obtained by a publicly knownprocess as mentioned above may be heated to 80-170° C. in the preheatingzone, and then stretched to a factor of 3.0-4.5 and more preferably3.2-4.2 with an infrared heating-type longitudinal stretching machine inthe machine direction at 120-180° C., more preferably 125-170° C. andeven more preferably 130-160° C. A roll-type longitudinal stretchingmachine may also be used, and the stretching is preferably conductedseveral different times in multistages in order to accomplish non-forcedlongitudinal stretching. The longitudinal stretching may be followed bytransverse stretching and, if necessary, heat setting and relaxationtreatment to obtain the desired PEN film.

The transverse stretching is preferably accomplished after furtherheating to 80-150° C. in a preheating zone after longitudinalstretching, followed by stretching in the transverse direction at120-180° C. in a stenter, to a factor of 3.0-4.5. A more preferredtransverse stretching temperature range is 125-170° C., and especially130-160° C. A more preferred transverse stretching factor range is from3.3 to 4.2. The heat setting is preferably carried out at 195-250° C.for 0.3-50 seconds. A more preferred heat setting temperature range is205-245° C. The heat relaxation treatment is preferably carried out at atemperature of 140-240° C. in the machine direction and/or transversedirection, to a relaxation factor in the range of 0.5-15%. Thestretching in the transverse direction may also be conducted severaldifferent times in multistages.

According to the production process for a membrane switch base film ofthe invention as described above (simultaneous biaxial stretching andsequential biaxial stretching), limiting the absolute value of thedifference between the melting sub-peak temperatures (Tsm) of the frontand back sides of the film (|Tsm (front)−Tsm (back)|) to no greater than6° C. requires verification and adjustment of the actual heatingtemperature of the upper surface and lower surface of the film in theheat setting zone. Here, adjustment does not refer merely to setting thetemperature to be equal on the upper and lower areas of the heat settingzone.

In the past, only the melting sub-peak temperature of the entire filmhas been considered important, without concentrating on the actual heattreatment temperature for heat setting on both the front and back sidesof the film. According to the invention, however, it has been discoveredthat in the case of a thick film having a biaxial stretched filmthickness of 50 μm or greater, the difference between the meltingsub-peak temperatures (Tsm) of the front and back sides of the film hasa major influence on the handling properties and workability in themembrane switch production steps, and a solution has been provided forthis problem.

In order to limit the refractive index to the range of 1.770-1.790 ineither or both the film processing direction and the widthwise directionof the film in the aforementioned production process for a membraneswitch base film of the invention (simultaneous biaxially stretching andsequential biaxially stretching), the temperature of the upper surfaceand lower surface of the film in each preheating zone beforelongitudinal stretching and transverse stretching may be adjusted to asatisfactory balance. Here, a satisfactory balance means that inconsidering the temperature difference on the upper surface and lowersurface of the film in each stretching zone, the temperature of theupper surface and lower surface of the film in the preheating zone isadjusted so as to allow non-forced stretching across the entire filmthickness. Thus, it does not mean simply setting the temperature to bethe same at the upper and lower areas of the preheating zone. Althoughthe temperature in the preheating zone has been given little attentionin the prior art, the present inventors have found that the temperaturein the preheating zone has a major influence on subsequent stretching ofthick films particularly having an unstretched film thickness of 700 μmor greater, or a uniaxially stretched film thickness of 300 μm orgreater.

As a general range for adjustment of the temperature difference betweenthe upper and lower areas of the preheating zone, the temperaturedifference between the upper and lower surfaces of the film in thepreheating zone is preferably no greater than 8° C., more preferably nogreater than 6° C. and even more preferably no greater than 5° C. Thetemperature in the preheating zone is preferably between 100° C. and160° C.

In order to adjust the absolute value of the difference in refractiveindexes of the front and back sides of the film to within the desiredrange, the temperature difference between the heat relaxation treatmenttemperatures in the machine direction and/or transverse direction afterheat setting on the upper and lower surfaces of the film may be adjustedto within 12° C., and preferably to within 7° C. For example, if theheat relaxation treatment temperature is set to be slightly higher onthe side with the smaller refractive index, when comparing therefractive indexes of the front and back side of the film, thedifference in refractive indexes on the front and back sides of the filmwill tend to be smaller.

The membrane switch base film of the invention obtained in this mannerexhibits excellent performance that is resistant to deterioration (dueto heat and moisture) in the harsh environments (high-temperature,high-humidity) of automobile interiors. Thus, the film of the inventionmay be suitably used as a film for automobile interiors which can lastfor the durable life of the automobile. Mechanism of the property ofresistance to such heat and moisture deterioration is associated withextreme orientation in at least one direction.

The membrane switch base film of the invention exhibits excellentdeformation recovery after being forcibly deformed for long periods andthen released from the deformation, and particularly it exhibitssatisfactory deformation recovery even at high temperatures of about 80°C. Thus, even when the film is embedded as a membrane switch base insidethe seat face of an automobile seat and subjected to the weight of asitting passenger, the film recovers from its deformation after thepassenger has left the seat, and can consistently function as a normalswitch. It is therefore particularly suited as a base material forpassenger seat sensor switches. That is, the film of the invention maybe suitably used as a membrane switch base film to serve as a sensorwhich detects when a passenger has sat on a seat, and/or as a sensor fordetection of the sitting position by detecting the pressure at differentlocations of the seat face after a passenger has sat thereon, when aplural of such switches are embedded inside the seat face of each of theseats other than the driver's seat of an automobile. The elasticstrength of films with extreme orientation in at least one direction isalso associated with expression of such deformation recovery.

EFFECT OF THE INVENTION

The membrane switch base film of the invention has excellent filmhandling properties and durability. It is particularly suitable formembrane switches used for automobile interior devices which arepotentially exposed to high temperature.

EXAMPLES

The present invention will now be explained in greater detail throughexamples. The property values throughout the examples were measured andevaluated by the methods described below. Unless otherwise specified,the parts and percentages in the examples are parts by weight andpercentages by weight.

(1) Measurement of ethylene-2,6-naphthalenedicarboxylate content (majorcomponent molar ratio, copolymerized component molar ratio)

The film sample was dissolved in a measuring solvent (CDCl₃:CF₃COOD=1:1)and then measured by ¹H-NMR and the integral ratio of each obtainedsignal was calculated.

(2) Metal concentration analysis

The titanium and phosphorus element concentrations were determined bysetting the dried film sample in a scanning electron microscope (SEM,Model S570 by Hitachi Instruments Service Co., Ltd.) and performingquantitative analysis with a connected energy dispersive X-raymicroanalyzer (XMA, EMAX-7000 by Horiba Co., Ltd.).

(3) Intrinsic viscosity (IV)

The intrinsic viscosity (IV) was measured at 35° C. using o-chlorophenolas the solvent.

(4) Film thickness

A micrometer (“Model K-402B”, trade name of Anritsu Corp.) was used formeasurement of the film thickness at 10 cm spacings in the lengthwiseand widthwise directions of the film, at a total of 300 locations. Theaverage value of the 300 measured film thicknesses was calculated as theoverall film thickness.

An electronic micrometer (“Model K-312A”, trade name of Anritsu Corp.)was also used for measurement along 2 m lengths in the lengthwise andwidthwise directions of the film, with a stylus force of 30 g and arunning speed of 25 mm/sec, to obtain a continuous thickness chart. Themaximum and minimum thicknesses were read from the chart and combinedwith the film thickness determined above to calculate the thicknessvariation according to the following formula.Thickness variation (%)=((Maximum thickness−minimum thickness)/filmthickness)×100

(5) Melting sub-peak temperature (Tsm) and difference between meltingsub-peak temperatures (Tsm) on front and back sides

One side of the film (front or back side) was sanded with sandpaper(#200) to 20% of the original film thickness. A DSC measuring filmsample was taken from the sanded film (front side sample or back sidesample), and a DSC220 differential scanning calorimeter by SeikoInstruments, Inc. was used to measure the sub-peak temperature with atemperature elevating rate of 20° C./min and a sample weight of 10 mgunder a nitrogen stream.

(6) Film refractive index

Using a laser refractometer based on the Abbe refractometer principle,the prism was contacted with each side of the film and the filmrefractive index was determined in each in-plane direction.Specifically, a prism coupler (Model 2010, by Metricon Corp.) was usedto measure the refractive index in the machine direction and thetransverse direction (nMD and nTD, respectively) at a wavelength of 633nm, on the front and back sides of the film. The absolute value of thedifference in refractive indexes on the front and back sides wasdetermined in the direction in which the value of the refractive indexwas between 1.770 and 1.790.

(7) Thermal shrinkage factor

The film was held in a tension-free state for 10 minutes in an ovenpreset to a temperature of 200° C., and the dimensional change beforeand after heat treatment was calculated from the following formula asthe thermal shrinkage factor.Thermal shrinkage factor %=(L0−L)/(L0)×100

Here, L0 represents the distance between gauge marks before heattreatment, and L represents the distance between gauge marks after heattreatment.

(8) Three-dimensional center plane average roughness (SRa)

This was measured using a surface roughness tester (“SURFCOM SE-3CK”,trade name of Tokyo Seimitsu Co., Ltd.), according to the methodspecified in JIS B-0601. Specifically, the peak profile of the filmsurface was measured under conditions with a measuring length (Lx) of 1mm, a sampling pitch of 2 μm, a cutoff of 0.25 mm, a thickness directionmagnification of 10,000×, a planar magnification of 200× and 100scanning lines (Ly=0.2 mm), and the surface roughness was calculated.

(9) Film density

The film density was measured by the suspension method at 25° C. in adensity gradient tube, using aqueous calcium nitrate as the solvent.

(10) Film continuous film processing property

The condition of film processing was observed after continuousprocessing of the film, and the time until local generation of stripedirregularity defects in the film processing direction of the film wasmeasured and evaluated on the following scale. Evaluations of 0 and Awere judged as acceptable.

∘: No generation of striped irregularity defects up to 72 hours afterthe start of film processing. Very satisfactory continuous filmprocessing property.

Δ: Generation of striped irregularity defects from 36 to 72 hours afterthe start of film processing. Generally satisfactory continuous filmprocessing property.

x: Generation of striped irregularity defects before 36 hours after thestart of film processing. Poor continuous film processing property.

(11) Film slipperiness

After stacking 400 sheets cut to A4 size from the film and placing themin the tray of a copy machine, 400 continuous copies were made onto OHPsheets, and the slipperiness of film feeding was evaluated on thefollowing scale. Evaluations of ∘ and Δ were judged as acceptable.

∘: Absolutely no film feeding failures, very satisfactory filmslipperiness.

Δ: 1-3 film feeding failures, generally satisfactory film slipperiness.

x: 4 or more feeding failures, problems with film slipperiness.

(12) Film workability

The film edge condition after punching or cutting was observed and theworkability was evaluated on the following scale. Films with evaluationsof ∘ and Δ were judged as acceptable for use according to the invention.

∘: Edges observed with microscope at 100× magnification after cuttingand punching. Condition of edges very linear and satisfactory with nodisturbances.

Δ: Edges observed with microscope at 100× magnification after cuttingand punching. Partial disturbances on edges, but irregularities of theedges could not be felt when stroked with the finger; edge conditiongenerally satisfactory for practical use.

x: Irregularities on edges after cutting and punching could be felt whenstroked with the finger; edge condition poor.

(13) Membrane switch durability evaluation

The membrane switch was subjected to a repeated ON/OFF test in anenvironment at 60° C., 65% RH. A procedure of applying a load to turnthe membrane switch ON (initial load: e.g. 1.5 kg/cm²) and then removingthe load, for 1 minute each, was repeated. This ON/OFF cycle wascontinued for 360 hours. Upon completion of the repeated ON/OFF test,the load was removed and the switch was allowed to stand for 30 minutesin an environment at 60° C., 65% RH. The load was then again applied tothe switch and the load required to turn the switch ON (post-test load)was measured. The sample was judged as acceptable if the post-test loadwas at least 90% of the initial load. This test was conducted with n=100and evaluation was made on the following scale (acceptability).Acceptability %=(number of samples with post-test load at least 90% ofinitial load/n number)×100

∘: Acceptability of ≧95%, switch durability very satisfactory.

Δ: Acceptability of ≧80%, switch durability generally satisfactory.

x: Acceptability of <80%, switch durability poor.

An evaluation of ∘ or Δ was considered having durability necessary for afilm of the invention.

(14) Overall Evaluation

The results of the evaluations described above were summarized in anoverall evaluation of “⊚”, “∘”, “Δ” or “x”. Evaluations of “⊚”-“Δ” werejudged as acceptable, while an evaluation of “x” was judged asunacceptable.

Example 1

Transesterification reaction was carried out according to an ordinarymethod using 100 parts of dimethyl 2,6-naphthalenedicarboxylate, 60parts of ethylene glycol and 0.03 part of manganese acetate tetrahydrateas a transesterification catalyst, with addition of 0.25 wt % of calciumcarbonate particles with a mean particle size of 0.5 μm, 0.06 wt % ofspherical silica particles with a mean particle size of 0.2 μm and 0.1wt % of spherical silica particles with a mean particle size of 0.1 μm,as lubricants, after which 0.042 part of triethyl phosphonoacetate wasadded and transesterification was allowed to proceed to substantialcompletion.

Next, 0.024 part of antimony trioxide was added, and polymerization wasconducted by an ordinary method at high temperature and high vacuum toobtain PEN having an intrinsic viscosity of 0.60 dl/g and a Tg of 121°C. The PEN polymer was dried at 175° C. for 5 hours and then supplied toan extruder, melted at a melt temperature of 300° C. and extruded fromthe die slit, and finally cooled to solidification on a casting drum setto a surface temperature of 55° C. to fabricate an unstretched film.

The unstretched film was stretched to a factor of 3.4 in the machinedirection at 140° C. It was then subjected to sequential biaxiallystretching to a factor of 3.8 in the transverse direction at 135° C.,and immediately subjected to heat setting for 6 seconds in a heatsetting zone adjusted to a temperature of 241° C. above the film and toa temperature of 239° C. below the film. The heat setting treatment wasfollowed by 1.5% heat relaxation treatment in the transverse directionto obtain a biaxially oriented film with a thickness of 100 μm, whichwas then wound up on a roll. The PEN base film was screen printed withsilver paste as a conductive circuit and with carbon paste for theprinting contacts (electrodes) and dried at 140° C. for 20 minutes toform a switch sheet, after which a film-like styrene-butadiene resin wasused as an adhesive for attachment of two of the sheets and a spacer forthe membrane switch. The properties, evaluation results and filmprocessing property of the obtained biaxially oriented film and theevaluation results of the membrane switch are shown in Table 1.

Example 2

The same procedure as in Example 1 was repeated, except that thetemperature above the film in the heat setting zone was 243° C. and thetemperature below the film was 238° C. The properties, evaluationresults and film processing property of the obtained biaxially orientedfilm and the evaluation results of the membrane switch are shown inTable 1.

Example 3

After adding 0.011 part of tetra-n-butyltitanate (TBT) and 0.25 wt % ofcalcium carbonate particles with a mean particle size of 0.5 μm, 0.06 wt% of spherical silica particles with a mean particle size of 0.25 μm and0.1 wt % of spherical silica particles with a mean particle size of 0.1μm, as lubricants, to a mixture of 100 parts of dimethyl2,6-naphthalenedicarboxylate and 56 parts of ethylene glycol, themixture was charged into an SUS (stainless steel) vessel suitable forpressure reaction and transesterification was conducted withpressurization at 0.07 MPa and temperature increase from 140° C. to 240°C., after which 0.042 part of triethyl phosphonoacetate (TEPA) was addedand transesterification was carried out to completion.

The reaction product was transferred to a polymerization vessel and thetemperature was raised to 290° C. for polycondensation reaction in ahigh vacuum of 100 Pa, to obtain PEN having an intrinsic viscosity of0.62 dl/g and a Tg of 121° C. The subsequent drying and film processingof the PEN polymer were accomplished by repeating the same procedure asin Example 1. The results are shown in Table 1.

COMPARATIVE EXAMPLE 1

A film was formed in the same manner as Example 1, except that thetemperature above the film in the heat setting zone was 220° C. and thetemperature below the film was 217° C. The results are shown in Table 1.

COMPARATIVE EXAMPLE 2

A film was formed in the same manner as Example 1, except that thetemperature above the film in the heat setting zone was 240° C. and thetemperature below the film was 232° C. The results are shown in Table 1.TABLE 1 Example 1 Example 2 Example 3 Comp. Ex. 1 Comp. Ex. 2 Meltingsub- Front ° C. 241.3 243.4 241.3 219.9 240.2 peak Back ° C. 239.2 238.2239.2 217.1 232.1 temperature |Front-back| ° C. 2.1 5.2 2.1 2.8 8.1(Tsm) Widthwise nTD 1.779 1.776 1.779 1.785 1.782 direction refractiveindex Titanium Type — — TBT — — compound Content mmol % — — 8 — —Phosphorus Type TEPA TEPA TEPA TEPA TEPA compound Content mmol % 48 4848 48 48 P/Ti Content ratio — — 6 — — Ti + P Content mmol % 48 48 56 4848 Thermal* MD % 0.6 0.4 0.6 1.3 0.8 shrinkage TD % 1.0 0.8 1.0 1.7 1.3factor 200° C., 10 min Intrinsic viscosity dl/g 0.60 0.60 0.62 0.63 0.63Film thickness μm 100 100 100 100 100 Film thickness variation % 4 6 4 35 Surface SRa nm 19 19 21 18 19 roughness Density g/cm³ 1.359 1.3601.359 1.353 1.357 Continuous film-processing Δ Δ ◯ Δ Δ property Filmworkability ◯ Δ ◯ X X Switch durability ◯ ◯ ◯ Δ ◯ Overall evaluation ◯ Δ⊚ X X

Example 4

Transesterification reaction was carried out according to an ordinarymethod using 100 parts of dimethyl 2,6-naphthalenedicarboxylate, 60parts of ethylene glycol and 0.03 part of manganese acetate tetrahydrateas a transesterification catalyst, with addition of 0.25 wt % of calciumcarbonate particles with a mean particle size of 0.5 μm, 0.06 wt % ofspherical silica particles with a mean particle size of 0.2 μm and 0.1wt % of spherical silica particles with a mean particle size of 0.1 μm,as lubricants, after which 0.042 part of triethyl phosphonoacetate wasadded and transesterification was allowed to proceed to substantialcompletion.

Next, 0.024 part of antimony trioxide was added, and polymerization wasconducted by an ordinary method at high temperature and high vacuum toobtain PEN having an intrinsic viscosity of 0.63 dl/g and a Tg of 121°C. The PEN polymer was dried at 175° C. for 5 hours and then supplied toan extruder, melted at a melt temperature of 300° C. and extruded fromthe die slit, and finally cooled to solidification on a casting drum setto a surface temperature of 55⁶C to fabricate an unstretched film.

The unstretched film was guided into a preheating zone at 120° C. beforelongitudinal stretching and heated with a temperature difference ofwithin 4° C. on the upper and lower surfaces of the film, after which itwas stretched to a factor of 3.2 in the machine direction at 145° C. Itwas then guided into a preheating zone at 130° C. before transversestretching and heated with a temperature difference of within 4° C. onthe upper and lower surfaces of the film, after which it was subjectedto sequential biaxial stretching to a factor of 3.9 in the transversedirection at 135° C., and immediately subjected to heat setting for 6seconds with a temperature of 237° C. above and below the film. The heatsetting treatment was followed by 1.5% heat relaxation treatment in thetransverse direction with a temperature of 215° C. above the film and atemperature of 218° C. below the film, to obtain a biaxially orientedfilm with a thickness of 100 μm, which was then wound up on a roll.

The PEN base film was screen printed with silver paste as a conductivecircuit and with carbon paste for the printing contacts (electrodes) anddried at 140° C. for 20 minutes to form a switch sheet, after which afilm-like styrene-butadiene resin was used as an adhesive for attachmentof two of the sheets and a spacer for the membrane switch. Theproperties, evaluation results and film processing property of theobtained biaxially oriented film and the evaluation results of themembrane switch are shown in Table 2.

Example 5

The same procedure as in Example 4 was repeated, except that thetemperature above the film for the heat relaxation treatment in thetransverse direction after heat setting was 213° C. and the temperaturebelow the film was 221° C. The properties, evaluation results and filmprocessing property of the obtained biaxially oriented film and theevaluation results of the membrane switch are shown in Table 2.

Example 6

After adding 0.011 part of tetra-n-butyltitanate (TBT) and 0.25 wt % ofcalcium carbonate particles with a mean particle size of 0.5 μm, 0.06 wt% of spherical silica particles with a mean particle size of 0.25 μm and0.1 wt % of spherical silica particles with a mean particle size of 0.1μm, as lubricants, to a mixture of 100 parts of dimethyl2,6-naphthalenedicarboxylate and 56 parts of ethylene glycol, themixture was charged into an SUS (stainless steel) vessel suitable forpressure reaction and transesterification was conducted withpressurization at 0.07 MPa and temperature increase from 140° C. to 240°C., after which 0.042 part of triethyl phosphonoacetate (TEPA) was addedand transesterification was carried out to completion.

The reaction product was transferred to a polymerization vessel and thetemperature was raised to 290° C. for polycondensation reaction in ahigh vacuum of 100 Pa, to obtain PEN having an intrinsic viscosity of0.61 dl/g and a Tg of 121° C. The subsequent drying and film processingof the PEN polymer were accomplished by repeating the same procedure asin Example 4. The properties, evaluation results and film processingproperty of the obtained biaxially oriented film and the evaluationresults of the membrane switch are shown in Table 2.

COMPARATIVE EXAMPLE 3

The same procedure as in Example 4 was repeated, except that stretchingto a factor of 3.3 in the machine direction was followed by stretchingto a factor of 3.4 in the transverse direction. The properties,evaluation results and film processing property of the obtainedbiaxially oriented film and the evaluation results of the membraneswitch are shown in Table 2.

COMPARATIVE EXAMPLE 4

The same procedure as in Example 4 was repeated, except that thetemperature above the film for the heat relaxation treatment in thetransverse direction after heat setting was 225° C. and the temperaturebelow the film was 211° C. The properties, evaluation results and filmprocessing property of the obtained biaxially oriented film and theevaluation results of the membrane switch are shown in Table 2.

COMPARATIVE EXAMPLE 5

The same procedure as in Example 4 was repeated, except that stretchingto a factor of 3.0 in the machine direction was followed by stretchingto a factor of 4.7 in the transverse direction. However, because offrequent tearing, film processing could not be continuously carried outfor more than an hour. As a result, no further evaluation was made ofthe film properties. TABLE 2 Example 4 Example 5 Example 6 Comp. Ex. 3Comp. Ex. 4 Comp. Ex. 5 Widthwise Top side (front) 1.780 1.775 1.7811.762 1.789 1.791 direction Bottom side (back) 1.785 1.787 1.784 1.7641.772 1.795 refractive index |Front-back| 0.005 0.012 0.003 — 0.0170.004 (nTD) Film-processing Top side (front) 1.738 1.738 1.740 1.7581.733 1.730 direction Bottom side (back) 1.736 1.736 1.737 1.755 1.7391.729 refractive index (nMD) Titanium compound Type — — TBT — — —Content mmol % — — 8 — — — Phosphorus Type TEPA TEPA TEPA TEPA TEPA TEPAcompound Content mmol % 48 48 48 48 48 48 P/Ti Content ratio 6 Ti + PContent mmol % 48 48 56 48 48 48 Intrinsic viscosity dl/g 0.63 0.63 0.610.63 0.63 0.63 Film thickness μm 100 100 100 100 100 100 Film thicknessvariation % 5 7 5 9 6 3 Surface roughness SRa nm 19 19 21 18 19 19Density g/cm³ 1.358 1.358 1.360 1.361 1.359 1.357 Continuousfilm-processing property Δ Δ ◯ Δ Δ X Film slipperiness ◯ ◯ ◯ ◯ ◯ — Filmworkability ◯ Δ ◯ ◯ X — Switch durability ◯ ◯ ◯ X ◯ — Overall evaluation◯ Δ ⊚ X X X

Example 7

Transesterification reaction was carried out according to an ordinarymethod using 100 parts of dimethyl 2,6-naphthalenedicarboxylate, 60parts of ethylene glycol and 0.03 part of manganese acetate tetrahydrateas a transesterification catalyst, with addition of 0.25 wt % of calciumcarbonate particles with a mean particle size of 0.5 μm, 0.06 wt % ofspherical silica particles with a mean particle size of 0.2 μm and 0; 1wt % of spherical silica particles with a mean particle size of 0.1 μm,as lubricants, after which 0.042 part of triethyl phosphonoacetate wasadded and transesterification was allowed to proceed to substantialcompletion.

Next, 0.024 part of antimony trioxide was added, and polymerization wasconducted by an ordinary method at high temperature and high vacuum toobtain PEN having an intrinsic viscosity of 0.60 dl/g and a Tg of 121°C. The PEN polymer was dried at 175° C. for 5 hours and then supplied toan extruder, melted at a melt temperature of 300° C. and extruded fromthe die slit, and finally cooled to solidification on a casting drum setto a surface temperature of 55° C. to fabricate an unstretched film.

The unstretched film was guided into a preheating zone at 120° C. beforelongitudinal stretching and heated with a temperature difference ofwithin 4° C. on the upper and lower surfaces of the film, after which itwas stretched to a factor of 3.2 in the machine direction at 145° C. Itwas then guided into a preheating zone at 130° C. before transversestretching and heated with a temperature difference of within 4° C. onthe upper and lower surfaces of the film, after which it was subjectedto sequential biaxial stretching to a factor of 3.9 in the transversedirection at 135° C., and immediately subjected to heat setting for 6seconds in a heat setting zone adjusted to a temperature of 241° C.above the film and to a temperature of 239° C. below the film. The heatsetting treatment was followed by 1.5% heat relaxation treatment in thetransverse direction with a temperature of 215° C. above the film and atemperature of 218° C. below the film, to obtain a biaxially orientedfilm with a thickness of 100 μm, which was then wound up on a roll.

The PEN base film was screen printed with silver paste as a conductivecircuit and with carbon paste for the printing contacts (electrodes) anddried at 140° C. for 20 minutes to form a switch sheet, after which afilm-like styrene-butadiene resin was used as an adhesive for attachmentof two of the sheets and a spacer for the membrane switch. Theproperties, evaluation results and film processing property of theobtained biaxially oriented film and the evaluation results of themembrane switch are shown in Table 3.

Example 8

The same procedure as in Example 7 was repeated, except that thetemperature above the film in the heat setting zone was 243° C. and thetemperature below the film was 238° C. The properties, evaluationresults and film processing property of the obtained biaxially orientedfilm and the evaluation results of the membrane switch are shown inTable 3.

Example 9

After adding 0.011 part of tetra-n-butyltitanate (TBT) and 0.25 wt % ofcalcium carbonate particles with a mean particle size of 0.5 μm, 0.06 wt% of spherical silica particles with a mean particle size of 0.25 μm and0.1 wt % of spherical silica particles with a mean particle size of 0.1μm, as lubricants, to a mixture of 100 parts of dimethyl2,6-naphthalenedicarboxylate and 56 parts of ethylene glycol, themixture was charged into an SUS (stainless steel) vessel suitable forpressure reaction and transesterification was conducted withpressurization at 0.07 MPa and temperature increase from 140° C. to 240°C., after which 0.042 part of triethyl phosphonoacetate (TEPA) was addedand transesterification was carried out to completion.

The reaction product was transferred to a polymerization vessel and thetemperature was raised to 290° C. for polycondensation reaction in ahigh vacuum of 100 Pa, to obtain PEN having an intrinsic viscosity of0.62 dl/g and a Tg of 121° C. The subsequent drying and film processingof the PEN polymer were accomplished by repeating the same procedure asin Example 7. The results are shown in Table 3. TABLE 3 Example 7Example 8 Example 9 Melting sub- Front ° C. 241.3 243.4 241.3 peak Back° C. 239.2 238.2 239.2 temperature |Front-back| ° C. 2.1 5.2 2.1 (Tsm)Widthwise Top side (front) 1.780 1.775 1.781 direction Bottom side(back) 1.785 1.787 1.784 refractive |Front-back| 0.005 0.012 0.003 indexFilm- Top side (front) 1.738 1.738 1.740 processing Bottom side (back)1.736 1.736 1.737 direction refractive index (nMD) Titanium Type — — TBTcompound Content mmol — — 8 % Phosphorus Type TEPA TEPA TEPA compoundContent mmol 48 48 48 % P/Ti Content ratio — — 6 Ti + P Content mmol 4848 56 % Thermal* MD % 0.6 0.4 0.6 shrinkage TD % 1.0 0.8 1.0 factor 200°C., 10 min Intrinsic viscosity dl/g 0.60 0.60 0.62 Film thickness μm 100100 100 Film thickness % 4 6 4 variation Surface Sra Nm 19 19 21roughness Density g/cm³ 1.359 1.360 1.359 Continuous film-processing Δ Δ◯ property Film slipperiness ◯ ◯ ◯ Film workability ◯ Δ ◯ Switchdurability ◯ ◯ ◯ Overall evaluation ◯ Δ ⊚

1. A membrane switch base film composed of a biaxially orientedpolyester film comprising polyethylene-2,6-naphthalenedicarboxylate asthe major component, said film being characterized by having arefractive index in the range of 1.770-1.790 on both surfaces in eitheror both the film processing direction and the widthwise direction and anabsolute value of no greater than 0.015 for the difference between therefractive indexes of both surfaces.
 2. A membrane switch base filmcomposed of a biaxially oriented polyester film comprisingpolyethylene-2,6-naphthalenedicarboxylate as the major component, saidfilm being characterized by having a melting sub-peak temperature ofbetween 220° C. and 250° C. as measured with a differential scanningcalorimeter (DSC) and an absolute value of no greater than 6° C. for thedifference between the melting sub-peak temperature on one surface andthe melting sub-peak temperature on the other surface.
 3. A membraneswitch base film composed of a biaxially oriented polyester filmcomprising polyethylene-2,6-naphthalenedicarboxylate as the majorcomponent, said film being characterized by having (1) a refractiveindex in the range of 1.770-1.790 on both surfaces in either or both thefilm processing direction and the widthwise direction and an absolutevalue of no greater than 0.015 for the difference between the refractiveindexes of both surfaces, and (2) a melting sub-peak temperature ofbetween 220° C. and 250° C. as measured with a differential scanningcalorimeter (DSC) and an absolute value of no greater than 6° C. for thedifference between the melting sub-peak temperature on one surface andthe melting sub-peak temperature on the other surface.
 4. A membraneswitch base film according to any one of claims 1 to 3, wherein therefractive index of the film in the widthwise direction is from 1.770 to1.790.
 5. A membrane switch base film according to any one of claims 1to 3 comprising a phosphorus compound and a titanium compound which issoluble in the polyethylene-2,6-naphthalenedicarboxylate, wherein thecontents of the titanium compound and phosphorus compound satisfy thefollowing inequalities (1) to (3).4≦Ti≦15  (1)0.5≦P/Ti≦15  (2)15≦Ti+P≦150  (3) (wherein Ti is the value of the number of moles oftitanium element in the titanium compound divided by the number of molesof the ethylene-2,6-naphthalenedicarboxylate component (mmol %), and Pis the number of moles of phosphorus element in the phosphorus compounddivided by the number of moles of theethylene-2,6-naphthalenedicarboxyiate component (mmol %).
 6. A membraneswitch base film according to claim 5, wherein the phosphorus compoundis a phosphonate compound represented by the following formula (I).R¹O—C(O)—X—P(O)—(OR²)₂  (I) (wherein R¹ and R² represent C₁₋₄ alkylgroups and X represents —CH₂— or —CH(Y)— (where Y represents phenyl)).7. A membrane switch base film according to claim 5, wherein thetitanium compound is a compound represented by the following formula(II) or the reaction product of a compound represented by formula (II)and an aromatic polyvalent carboxylic acid represented by the followingformula (III).Ti(OR³)(OR⁴)(OR⁵)(OR⁶)  (II) (wherein R³, R⁴, R⁵ and R⁶ eachindependently represent C₂₋₁₀ alkyl groups and/or phenyl).

(wherein n represents an integer of 2 to 4).
 8. A membrane switch basefilm according to any one of claims 1 to 3, wherein the thickness isfrom 40 μm to 190 μm.
 9. A membrane switch base film according to anyone of claims 1 to 3, wherein the surface roughness (SRa) of at leastone surface is from 10 nm to 45 nm.
 10. A membrane switch base filmaccording to any one of claims 1 to 3, wherein the thermal shrinkagefactor in the film processing-direction and widthwise direction when itis subjected to heat treatment at 200° C. for 10 minutes is between 0.2%and 1.4%.
 11. A membrane switch base film according to any one of claims1 to 3, which is used in an automobile interior.
 12. A membrane switchbase film according to claim 11, wherein the membrane switch is used asa sensor in each seat in an automobile to detect when a passenger hassat thereon, with a plural of such switches being embedded inside eachseat face.
 13. A membrane switch base film according to claim 11,wherein the membrane switch is used as a sensor for detection of thesitting position by detecting the pressure at different locations of theseat face of each seat in an automobile after a passenger has satthereon, with a plural of such switches being embedded inside the seatface of each seat.
 14. A membrane switch characterized by comprising amembrane switch base film according to any one of claims 1 to 3, aspacer and electrodes.